Multistage fluidized beds may be used as calciners, for waste-heat recovery, and as coal-combustion boilers, among other applications. In multistage fluidized beds, gases flow in sequence through a first bed and then one or more additional beds. A fluidized bed is a vessel with a distributor plate through which gases flow, and has a granular material, such as sand or limestone, placed on the distributor plate.
If the gases flowing through the fluidized bed entrain sticky materials, such as coal melted ash, the perforations in the subsequent distributor plates may become plugged, requiring the fluidized bed to be shut down for clean out. If the need for cleanout becomes too frequent, the use of multiple stages becomes unfeasible. Multi-bed coal-fired fluidized bed combustors with conventional distributor designs typically can operate for about two months between cleanouts. A distributor must also be able to prevent weepage, or, the flow of bed solids through the distributor when the gasflow is turned off.
Distributors of various designs for distributing gasflow uniformly across a fluidized bed are known. In the drilled-plate design, numerous small holes are drilled in a horizontal plate. Since the holes in drilled-plate distributors are vertical, small holes (relative to the bed-solid particles) must be used to prevent weepage. The smaller the holes, however, the more susceptible the plate is to pluggage by sticky materials. A beehive-shaped buildup forms at the entrance of each hole, increasing the blockage until shutdown is required. Such buildups have been experienced at temperatures at which the material are normally non-sticky, as with coal ash upwards of 1000.degree. F. below the ash fusion point. Even at such temperatures, buildups requiring shutdown have been experienced within minutes of the start of operation. The drilled-plate distributor may therefore be unsuitable for use with multistage fluidized beds using sticky materials.
Other distributor designs use bubblecaps to control the gasflow. Bubblecap distributors consist of arrays of vertical pipes mounted in holes in a horizontal plate. To form the bubblecaps, the pipes are capped at the top, and holes or orifices that are approximately horizontal are provided near the top of the pipe to control the gasflow. The use of bubblecaps avoids the spouting of gasflow through the bed, and prevents weepage. Some bubblecaps provide additional baffles or shields to increase the protection against weepage. Because they can use larger holes without incurring weepage, the hole size of bubblecap distributors can be half an order of magnitude larger than with drilled-plate designs, reducing the tendency to pluggage.
Buildups still occur in bubblecap distributors, but at a much lower rate than with drilled-plate designs. The buildups in bubblecap distributors, however, form within the bubblecaps themselves. These buildups typically are dense and impervious, and are found at the entrance to the orifices, where the gases change direction. They form by the impact of solids that can't turn as sharply as the gases, hitting the bubblecap instead of leaving at the hole. Impact causes the materials to be sticky under conditions where they are nonsticky at low velocities, e.g., at the bed surface and walls. No buildup occurs, however, at the entrance to the bubblecaps, i.e., at the pipe inlet. For impact-caused buildup to occur with an ash-laden fluidized bed, two factors must be present: high velocity, and a change of direction of the gas stream in the bubblecap.